U.S. patent number 4,237,897 [Application Number 05/957,411] was granted by the patent office on 1980-12-09 for battery life extender.
This patent grant is currently assigned to Pacesetter Systems, Inc.. Invention is credited to Russell R. Beane, Brian M. Mann.
United States Patent |
4,237,897 |
Beane , et al. |
December 9, 1980 |
Battery life extender
Abstract
A battery life extender for use in an implantable tissue
stimulator. The tissue stimulator comprises a battery which powers
a volatile memory, control circuits and a pulse output circuit all
of which are connected in parallel across the battery. The
invention provides a means for controlling current through the
output circuit as a function of a difference voltage between the
battery output voltage and a reference voltage. The reference
voltage and battery voltage comprise inputs to a differential
amplifier which in turn provides an output control voltage which
controls a field-effect transistor (FET) connected in series with
the output circuit and battery. As this control voltage rises,
current through the field-effect transistor is reduced, thereby
reducing the amplitude of the pulses from the output circuit and
the current drain from the battery. This keeps the battery output
voltage high and maintains a more nearly constant voltage on the
volatile memory and control circuits for a longer period of time
than would otherwise be available.
Inventors: |
Beane; Russell R. (Sepulveda,
CA), Mann; Brian M. (Northridge, CA) |
Assignee: |
Pacesetter Systems, Inc.
(Sylmar, CA)
|
Family
ID: |
25499531 |
Appl.
No.: |
05/957,411 |
Filed: |
November 3, 1978 |
Current U.S.
Class: |
607/34 |
Current CPC
Class: |
A61N
1/378 (20130101); G05F 1/56 (20130101) |
Current International
Class: |
A61N
1/378 (20060101); A61N 1/372 (20060101); G05F
1/56 (20060101); G05F 1/10 (20060101); D61N
001/36 () |
Field of
Search: |
;128/419PS,419PG |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Freilich, Hornbaker, Wasserman,
Rosen & Fernandez
Claims
What is claimed is:
1. In an implantable human tissue stimulator having a volatile
memory and control circuit means and an output circuit both of
which are connected in parallel across a battery, said memory and
control circuit means comprising a first resistance and said output
circuit comprising a second resistance, the improvement comprising
means for altering the ratio of current flowing through said second
resistance with respect to current flowing through said first
resistance as a function of said battery output voltage.
2. The improved stimulator of claim 1 in which said means for
altering comprises:
means for generating a substantially constant reference
voltage;
means for providing a control voltage related to a voltage
difference between said reference voltage and said battery output
voltage; and
means for reducing current through said second resistance as a
function of said control voltage.
3. The improved stimulator of claim 2 in which said means for
reducing comprises:
a transistor in series with said second resistance and said
battery; and
means for connecting the control electrode of said transistor to
said control voltage whereby said control voltage controls current
through said transistor.
4. The improved stimulator of claim 3 in which said transistor
comprises a field-effect transistor (FET).
5. The improved stimulator of claim 1 in which said altering means
comprises:
means for providing a control voltage related to the output voltage
of said battery; and
means for reducing current through said second resistance as a
function of said control voltage.
6. The improved stimulator of claim 5 wherein said means for
providing comprises:
a control transistor;
means for applying a voltage related to said battery voltage to the
base of said control transistor whereby current flow through said
control transistor is related to said battery voltage; and
means for generating a voltage related to said current flow through
said control transistor, said voltage being related to said control
voltage.
7. The improved stimulator of claim 6 wherein said control
transistor is an npn transistor and said means for generating
comprises:
a dropping resistor connected in series between said control
transistor collector and a positive terminal of said battery;
and
means for connecting said control transistor collector to a return
terminal of said battery whereby the voltage at said control
transistor collector comprises said voltage related to said current
flow.
8. The improved stimulator of claim 7 wherein said means for
reducing current comprises an FET in series with said second
resistance.
9. In an implantable human tissue stimulator comprising a volatile
memory and control circuit means having a first resistance and an
output circuit having a second resistance, both of which are
connected in parallel across a battery, a method of varying the
ratio of current through said second resistance with respect to
current through said first resistance, the steps comprising:
generating a substantially constant reference voltage;
providing a control voltage related to a voltage difference between
said battery voltage and said reference voltage; and
controlling current through said second resistance as a function of
said control voltage.
10. The method of claim 9 in which said controlling step further
comprises the step of biasing a transistor with said control
voltage, said transistor being in series with said second
resistance and said battery.
11. In an implantable human tissue stimulator having a voltage
source driving a first load impedance and a second load impedance
in parallel with said first load impedance, the improvement
comprising:
means for generating a reference voltage;
means for providing a control voltage related to a voltage
difference between said reference voltage and said voltage source;
and
means for controlling current through said second load impedance as
a function of said control voltage.
12. The improved stimulator of claim 11 wherein said voltage source
is a dc voltage source having positive and negative terminals and
said means for generating comprises:
a resistor; and
a diode, said resistor and said diode being connected in series
across said voltage source so that a voltage drop across said diode
comprises said reference voltage.
13. The improved stimulator of claim 11 wherein said means for
providing comprises a differential amplifier having a voltage
related to said voltage source output voltage as a first input and
said reference voltage as a second input, said differential
amplifier providing an output voltage related to a difference
between said first and second input voltages, said output voltage
comprising said control voltage.
14. The improved stimulator of claim 11 wherein said means for
controlling comprises a transistor in series with said second load
impedance and said voltage source, and having its base electrode
connected to said control voltage whereby current through said
transistor is related to said control voltage.
15. The improved stimulator of claim 14 wherein said transistor is
a field-effect transistor (FET).
Description
FIELD OF THE INVENTION
The invention relates to circuitry for extending the life of a
battery in an implantable tissue stimulator and more particularly
to circuitry which selectively controls current through one portion
of the stimulator as a function of battery output voltage.
BACKGROUND OF THE INVENTION
A typical implantable tissue stimulator such as a device for
electrically stimulating the heart at predetermined time intervals
comprises a battery, an output circuit for providing tissue
stimulation pulses and a volatile memory and control circuit for
controlling frequency and other parameters provided by the tissue
stimulator. It is well known in the medical sciences that the
frequency provided by a tissue stimulator is extremely critical to
the health of a patient. In programmable tissue stimulators these
pulses are controlled by a programmable memory which contains
information predetermined with respect to pulse requirements of a
particular patient. A problem with these system utilizing a
volatile memory is that the information contained therein can be
affected if the battery voltage drops below a certain critical
level, this altered information having a deleterious effect on a
patient due to its control of the tissue stimulation pulse
characteristics. Conventional tissue stimulators have attempted to
alleviate the problem in several ways, one of which has been to
inactivate the output circuit if the battery output voltage falls
below a predetermined level. However this results in an immediate
removal of pulses which in severe cases could result in extreme
discomfort and even death to the patient prior to the time that the
battery could be removed and replaced.
SUMMARY OF THE INVENTION
The present invention solves the above problems by providing
circuitry whereby a substantially constant voltage is supplied to
the volatile memory for a longer period of time by gradually
reducing the amplitude of output pulses from the output circuit,
thereby taking advantage of a safety factor associated with
selection of a suitable pulse amplitude.
In an electronic circuit having a voltage source driving a first
load impedance and a second load impedance in parallel with the
first load impedance, the invention provides a means for generating
a reference voltage, a means for providing a control voltage
related to a voltage difference between the reference voltage and
the source voltage, and a means for controlling current through the
second load impedance as a function of the value of the control
voltage. In a specific embodiment, the electronic circuit is
contained within an implantable tissue stimulator, the voltage
source is a battery, the first load impedance is a volatile
programmable memory and control circuity, and the second load
impedance is an output circuit. The reference voltage is generated
by an appropriate circuit and the control voltage is obtained from
the output of a differential amplifier having the reference voltage
and a predetermined fraction of the battery voltage as inputs. The
control voltage output is applied to the gate or control electrode
of a field-effect transistor (FET) connected in series with the
second load impedance so that as the control voltage or difference
voltage from the differential amplifier increases the current
through the second impedance decreases thereby causing the battery
voltage to increase due to a smaller voltage drop across the
internal impedance of the battery. Thus the invention provides a
means to maintain a substantially constant voltage across the first
impedance (volatile memory) at the expense of reduced current flow
through the second impedance (output circuit).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a circuit according to a first
embodiment of the present invention;
FIG. 2 is a graph showing battery voltage versus time with and
without a circuit according to the present invention;
FIG. 3 is a graph showing current through the output circuit versus
time when utilizing a circuit according to the present invention;
and
FIG. 4 is a schematic diagram of a further embodiment of the
invention showing an alternate means of generating the control
voltage.
DETAILED DESCRIPTION
As required, detailed illustrative embodiments of the invention are
disclosed herein. These embodiments exemplify the invention and are
currently considered to be the best embodiments for such purposes.
However, it is to be recognized that other means for generating a
reference voltage and other means for controlling current through
the output circuit as a function of a voltage differential between
the reference voltage and battery voltage could be utilized.
Accordingly, the specific embodiments disclosed are representative
in providing a basis for the claims which define the scope of the
present invention.
As previously explained, the invention provides a means for
controlling the current through an output circuit portion of an
implantable human tissue stimulator in order to maintain a
predetermined voltage differential across a volatile memory and
control circuit (hereinafter referred to as the volatile memory)
for as long a period of time as possible, the volatile memory and
output circuit being connected in parallel across a battery. The
invention discloses a means whereby a reference voltage derived
from the battery is compared to the battery output voltage, a
voltage proportional to their difference being used to bias a
field-effect transistor (FET) connected in series with the output
circuit. Thus as the voltage differential between the battery and
the reference voltage decreases, the FET is biased so as to reduce
the current flowing therethrough, thereby reducing current drain
from the battery and causing the battery voltage across the
volatile memory to remain constant.
Referring to FIG. 1, an implantable tissue stimulator 10 comprises
a battery 12, a volatile memory 14 and output circuit 16, the
volatile memory 14 and output circuit 16 being connected in
parallel across the battery 12. The volatile memory 14 can be
functionally represented with respect to the battery as a first
resistance 14' and the output circuit 16 can be represented as
second resistance 16'. As has been previously explained, the
volatile memory 14 comprises a programmable memory which determines
the frequency, duration and other characteristics of tissue
stimulation pulses from the output circuit 16, the pulses appearing
on an output line 18. The parameters describing the pulses are
determined in accordance with programmed instructions provided to
the volatile memory 14, and the amplitude of the pulses is
determined by the voltage V.sub.O across the output circuit 16. As
is well known in the medical sciences the repetition rate of the
pulses is critical with respect to a patient and must meet
predetermined criteria; however the amplitude of the pulses is
normally set higher than needed in order to provide a safety factor
for variations in the stimulation threshold of the living tissue.
Thus a predetermined amplitude for the tissue stimulation pulses is
established; however the pulses can be above that predetermined
level without harming the patient. It is essential however, that
the voltage across the volatile memory 14 not fall below a
predetermined level in order to maintain integrity of the
pulse-determining parameters stored within the volatile memory 14.
In a typical tissue stimulator, the output circuit 16 draws
approximately 10 times as much current as the volatile memory 14
(I.sub.O .apprxeq.10 I.sub.M). If the battery 12 output voltage
V.sub.B begins to drop, reduction of the current I.sub.O through
the output circuit 16 will reduce the battery current I.sub.B
through an internal resistance 20 of the battery, thereby causing
the output voltage V.sub.B of the battery 12 appearing across the
volatile memory 14 to increase.
Control of the current through the output circuit 16 as a function
of the output voltage V.sub.B of the battery 12 is accomplished as
follows. Referring again to FIG. 1, a reference voltage generator
22 comprising a forward biased diode 24 and a dropping resistor 26
is provided. A reference current I.sub.R which flows through the
dropping resistor 26 and the diode 24 results in a substantially
constant voltage drop across the diode 24, this voltage drop being
represented by V.sub.R and being present on a reference voltage
line 28. A differential amplifier 32 is provided, the differential
amplifier 32 having a predetermined fraction of the battery output
voltage V.sub.B present on output line 34 and as the other input
the output V.sub.R from the reference voltage generator 22. Two
biasing resistors 36 and 37 are chosen so that a battery voltage
substantially equal to that of the minimum voltage required for
proper operation of the volatile memory 14 results in a bias
voltage V.sub.BV being substantially equal to the voltage drop
across the diode 24. The differential amplifier 32 provides a
control voltage output V.sub.C which appears on an output line 38,
the control voltage V.sub.C being a function of the voltage
differential between the reference voltage V.sub.R and the bias
voltage V.sub.BV. A P channel enhancement mode field-effect
transistor (FET) 42 is connected in series between the output
circuit 16 and the battery 12 although other types of transistors
or circuits could be utilized. The gate electrode 44 of the FET 42
is connected to the differential amplifier output line 38. In the
circuit described, the field-effect transistor 42 is acting as a
variable resistance, the resistance increasing as the voltage
differential between V.sub.BV and V.sub.R decreases.
In operation, continued use of the battery results in a gradual
decrease in battery voltage V.sub.B due to a gradual increase in
the internal resistance 20 of the battery 12. When V.sub.BV is
higher than V.sub.R, the output of the differential amplifier 32 is
low, thereby keeping the FET 42 on. However, as soon as the value
of V.sub.BV drops to a level substantially equal to that of
V.sub.R, then the output of the differential amplifier 32 begins to
rise, thereby beginning to turn the FET 42 off so that the current
I.sub.O flowing therethrough is reduced. This lowered I.sub.O
lowers I.sub.B (I.sub.B =I.sub.M +I.sub.O) and thereby reduces the
voltage drop due to the internal impedance 20 of the battery and
causes V.sub.B to remain high. Thus the output voltage V.sub.B of
the battery is maintained somewhat constant as a result of
gradually reducing I.sub.O which in turn reduces the amplitude of
the output pulses from the output circuit 16. Thus V.sub.B is
maintained at a higher level than it normally would be due to this
reduced current through the output circuit 16.
The above-described effects can be seen diagrammatically by
referring to FIGS. 2 and 3. As can be seen, during the first
portion 50 of the battery voltage versus time curve the battery
voltage V.sub.B decreases slightly as a function of time. At time
T.sub.1 and in the absence of a circuit as described above, the
battery voltage V.sub.B would continue to decrease as shown by the
dotted line 52. However by using the above described circuit, the
battery voltage V.sub.B during the time between T.sub.1 and T.sub.2
remains substantially constant as shown at 54 because of the
continually reduced current flowing to the output circuit 16 as
represented by the current line 56 shown in FIG. 3. If the FET 42
is completely biased to cut off by the control voltage V.sub.C so
that no current flows therethrough as indicated at time T.sub.2,
then the battery voltage V.sub.B continues to drop, however at a
slower rate due to the lower current drain required by the volatile
memory 14 as shown at 58.
Attention is now directed to FIG. 4 wherein a further embodiment of
the invention is shown. First and second dropping resistors 70 and
71, respectively, are connected across the battery 12, their
junction point 72 being connected to the control electrode of a
transistor Q1. The emitter electrode of transistor Q1 is connected
to ground. The collector electrode is connected to the battery 12
through a resistor 73, and to the input of an amplifier 75 whose
output V.sub.C controls the FET 42.
In operation, dropping resistors 70 and 71 are chosen so that if
V.sub.B is not less than a level as that represented in FIG. 2 at
time T.sub.1, Q1 is fully conductive. Therefore, the output of
amplifier 75 is low, i.e., V.sub.C is low and the FET 42 is fully
conductive. However, when the voltage V.sub.B is substantially
equal to or below that represented in FIG. 2 at time T.sub.1, the
voltage at the junction 72 of dropping resistors 70 and 71
decreases. As a result Q1 is driven toward cutoff, resulting in an
increased input to the amplifier 75. Thus V.sub.C increases,
thereby reducing I.sub.O through the FET 42 and the output circuit
16.
Thus as one can appreciate, a circuit suitable for use in an
implantable tissue stimulator has been described whereby the life
of a volatile memory stimulator is extended at the expense of a
safety factor in the amplitude of an output tissue stimulation
pulse.
* * * * *